Several behavioral and neurological disorders are presently attributed to pathologies of specific neurotransmitter systems. These systems include (1) the dopaminergic nigrostriatal pathway, degeneration of which results in Parkinson's disease, (2) projections from serotonergic dorsal raphe nucleus, which are thought to play a role in the generation of anxiety, and (3) the widely projecting cholinergic neurons in the nucleus basalis that degenerate in individuals afflicted with Alzheimer's disease. In view our present technological capabilities and the enormous complexity of the mammalian brain, it has frequently proven fruitful to study such systems in the experimentally favorable neural circuits found in certain invertebrates. In these simpler systems, it is often possible to relative the activity of specific identified neurons directly to the behavior of the organism. The proposed investigation is part of an interdisciplinary approach to the study of neurotransmitter systems in the regulation of complex natural behavior patterns in a novel model system, the marine mollusc Bursatella leachii. Preliminary studies indicate that feeding in this organism requires the co-activation and coordination and coordination of two distinct central pattern generator circuits (biting and locomotion) and that both of these circuits are regulated by a system of interneurons in which gamma-aminobutyric acid (GABA) acts a neurotransmitter. The specific aims of the proposed studies are: (1) Observe and quantify Bursatella feeding behaviors in the native habitat and in the laboratory (methods: ecological census techniques, underwater photography), (2) Characterize the biting and locomoter systems that produce feeding-related behaviors (methods: in vivo and in vitro electrophysiology, dye injection), (3) Examine the role of a recently identified commissural GABAergic system in coordinating the bilateral components of the biting and locomotor circuits (methods: intracellular electrophysiology, dye injection in combination with immunohistochemistry), 4) Examine the role of GABAergic, interganglionic "command" or regulatory neurons in the control of the biting and locomoter circuits. These studies are ultimately designed to test the hypothesis that specific interneuronal transmitter systems serve to convey motivation or drive states to relevant motor circuits.